1. Field of the Invention
The present invention relates to a specimen analysis system which analyzes a characteristic of a specimen by injecting light into the specimen and acquiring information carried by light which propagates in and exits from the specimen.
2. Description of the Related Art
In recent years, the development of light absorption analysis (spectrometry) of a light-scattering substance such as a biological substance has been proceeding. The light applied to a light-scattering substance such as a biological substance undergoes multiple scattering and absorption in the light-scattering substance, and exits from the light-scattering substance. The behavior of light in a light-scattering substance can be expressed by an optical diffusion equation based on the optical diffusion theory, and the optical diffusion equation can be expressed as a differential equation. Therefore, it is possible to obtain a distribution of values of an optical characteristic such as an absorption coefficient, a scattering coefficient, or the like of a biological substance by measuring light which exits from the biological substance, and substituting the measured values in the optical diffusion equation.
For example, systems using the time-resolved spectroscopy or systems using the frequency-domain spectroscopy have been proposed as systems in which a distribution of optical characteristic values is obtained as above. The time-resolved spectroscopy uses ultra-short pulsed light having a width of approximately a picosecond, and the frequency-domain spectroscopy uses high-frequency modulated light.
According to the time-resolved spectroscopy, it is possible to obtain a distribution of optical characteristic values in a specimen of a light-scattering substance on the basis of the optical diffusion equation by injecting pulsed light into the specimen, and measuring the time spread (time profile) of the pulsed light which exits from the specimen after propagation through the specimen, where the time spread is caused by scattering of the pulsed light in the specimen.
According to the frequency-domain spectroscopy, it is possible to obtain a distribution of optical characteristic values in a specimen of a light-scattering substance on the basis of the optical diffusion equation by injecting high-frequency modulated light into the specimen, and measuring the intensity variation and phase delay at the modulation frequency in the light which exits from the specimen after propagation through the specimen, as disclosed by M. Vauhkonen et al., in “Utilizing the radiative transfer equation in optical tomography,” OSA Biomedical Optics, pp. WF48-50, 2004.
However, when light is injected into a specimen as a biological substance or the like, and a distribution of optical characteristic values in the specimen is calculated by using the optical diffusion equation, the accuracy of the calculation deteriorates since the light is forward scattered in the vicinity of the incident point of the light. As disclosed by L. Marti-Lopez et al., “Interpretation of the failure of the time-independent diffusion equation near a point source,” Optics Communications, vol. 242, pp. 23-43, 2004 a technique for solving the above problem has been proposed. According to this technique, a radiative transfer equation, instead of the optical diffusion equation, is used for calculating a distribution of optical characteristic values or the like in a vicinity of the incident point of the light in a specimen. For example, when the radiative transfer equation is used in calculation based on measurement results obtained from a near-incident-point region at distances less than 5 mm from the incident point of the light, and the optical diffusion equation is used in calculation based on measurement results obtained from farther regions at distances equal to or greater than 5 mm from the incident point of the light, it is possible to increase the accuracy in calculation of the distribution of the optical characteristic values or the like in the entire volume of the specimen including the vicinity of the incident point of the light (i.e., the near-incident-point region) and the other regions far from the incident point.
Nevertheless, the radiative transfer equation is much more complex than the optical diffusion equation. Therefore, when the radiative transfer equation is used in calculation of a distribution of optical characteristic values or the like in a specimen, the calculation time increases.